Semiconductor devices rely for the most part on the properties of germanium, silicon, and gallium arsenide materials. These materials are subject to significant molecular diffusion at temperatures as low as 100.degree. C. Such diffusion tends to adversely affect the integrity of the microscopic patterns required for proper operation of the semiconductor device. Such semiconductor devices are minuscule and delicate, and must be protected against the environment by a device carrier such as a metal can, or a plastic or ceramic encapsulant. The device carrier acts as a barrier which tends to impede heat transfer away from the semiconductor device. As the complexity and speed of semiconductor devices has increased, the internal heat generated by such semiconductor devices has tended to increase. In many applications, simple "heat sinks" which transfer heat from the semiconductor device carrier to the ambient air may be sufficient to maintain the temperature of the semiconductor device at a level providing satisfactory performance and reliability. In some applications such as personal computers, recent microprocessors generate sufficient heat that an integral heat-sink/air-circulating fan is required for reliable performance. In some applications, however, which include industrial and military applications, the ambient air temperature may be high enough so that sufficient heat cannot be effectively transferred from the device carrier by convection alone, and/or fan reliability itself is not high enough. Also, the air may be contaminated in which case it cannot be allowed to come into contact with the electronic interconnections. In those cases, direct conduction of the heat to a remote heat sink may be required.
In FIG. 1, an electronic assembly 10 includes a peripheral, thermally conductive frame 12. Frame 12 defines an enclosed space 14, which is occupied, at least in part, by a printed circuit board or printed wiring board 16, the upper surface 16u of which is visible. A plurality of electrical or electronic components, some of which are designated as 18a and 18b, are mounted on the upper or component side 16u of board 16. The frame 12 of the assembly 10 defines a pair of flanges 22, only one of which is visible, which allow the assembly to be easily slid into position in a rack (not illustrated) or other holder. A set of "card locks" 26 extends along at least a portion of the upper surfaces of flanges 22 to provide clamp pressure for tending to hold the flanges in position in the rack. A "mezzanine" printed-circuit board 20 is mounted above some of the components 16. A pair of electrical connectors 24a, 24b provide electrical interconnection between the printed circuit boards 16 and 20 and related external electronic circuitry (functions). The assembly 10 also may include covers (not illustrated), which protect the printed-circuit boards and the components. The presence of the mezzanine board makes space in the assembly critical.
FIG. 2 is a cross-section of a portion of the assembly of FIG. 1 near the frame, showing how the flanges connect to the surrounding rack or frame, and illustrating heat conduction paths. In FIG. 2, the rack is illustrated in cross-section as 210, and includes bosses or protuberances 210p1 and 210p2, which provide support for the illustrated edge of the module or assembly 10. As illustrated, flange 22 has a lower surface lying against a surface of boss 210p2, and card lock 26 extends between the upper surface of flange 22 and the lower surface of upper boss 210p1. In this context, the terms "upper" and "lower" refer to position as referred to the FIGURE, and do not necessarily relate to the position of the actual mounted assembly 10.
In FIG. 2, component or device 18a is electrically and mechanically connected to the upper surface 16u of printed circuit board 16 by electrically conductive wires or leads 19. The lower surface 161 of printed circuit board 16 is visible in FIG. 2. There will always be some space or gap 209, however small, between the lower surface of component 18a and the upper surface 16u of printed circuit board 16. The gap dimension is exaggerated in FIG. 2 for clarity. During operation of the component 18a, heat is necessarily generated. If the heat were allowed to accumulate, the temperature of the component would rise continuously, until it was destroyed. Heat may be removed or transferred away from the component by radiation, convection, or conduction, as known. In the illustrated context, heat removal by radiation and convection may not be sufficient to maintain the component at a temperature within its reliable operating range. Additional heat transfer is provided by thermal conduction through gap 209 between component 18a and the upper surface of printed circuit board 16u, as illustrated by dashed arrow 220. The heat entering the upper surface of printed circuit board 16 flows laterally, as indicated by dash arrow 222, and through the joint between the upper surface 16u of printed wiring board 16 and frame 12, as indicated by dash arrow 224. Within frame 12, the heat which enters from printed circuit board 16 flows by a short path through the interface with lower boss 210p2 of rack 210. Rack 210 is deemed to be a heat sink, meaning that it is large enough, or has sufficient surfaces for loss of heat, so that it remains at a relatively constant or low temperature. Those skilled in the art know that the printed circuit board 16 is not a good conductor of heat, and gap 209 tends to impede heat transfer, and that, as a consequence, the temperature rise of components such as 18a may still exceed that desired.
FIG. 3 is a simplified cross-section of another arrangement, similar to that of FIG. 2, but which provides superior conductive heat transfer. Elements of FIG. 3 which correspond to those of FIG. 2 are designated by like reference numerals. FIG. 3 differs from FIG. 2 in that it includes an upper or top heat transfer plate 316, which is supported by frame 12, and fastened thereto by a set of screws, one of which is illustrated as 330. An efficient thermally conductive heat transfer device or pad 318 is located between the upper surface of component 18a and the lower surface 316l, to improve the thermal conduction of what would otherwise be an air gap of poor thermal conductance. In addition to heat transfer as described in conjunction with FIG. 2, as illustrated by dash arrows 220, 222, 224, and 226, the presence of the heat transfer plate 316 and device 318 provides additional paths including the path between the upper surface of the component 18a and the lower surface 316l of heat transfer plate 316 by way of device 318, which is illustrated by dash arrow 320, a lateral path through heat transfer plate 316, as illustrated by dash arrow 322, and a further path through the juncture between heat transfer plate 316 and frame 12, as illustrated by dash arrow 324. All of the heat entering frame 12, and represented by arrows 224 and 324, is conducted to boss 210p2, as suggested by dash arrow 226. The arrangement of FIG. 3 provides superior heat transfer by comparison with the arrangement of FIG. 2, because there are two parallel paths for transfer of heat over much of the path, and because the additional path through heat transfer plate 316 is likely to have greater capacity for thermal conduction than the path through printed circuit board 16.
One problem with the arrangement of FIG. 3 is that the device 318 must fill the gap between the lower surface 316l of the heat transfer plate 316 and the upper surface of the component 18a. The dimension of this gap cannot be minimized because of tolerance build-ups, and possibly because of the presence of a mezzanine circuit board as illustrated in FIG. 1, or because of other mechanical constraints, as might occur if the components to be heat sunk have different heights above the printed-circuit board. Another problem with the arrangement of FIG. 3 is that the thermal path from the component 18a to the lower surface 316l of heat transfer plate 316 must be capable of disassembly without damage to component 18a, because the heat transfer plate is likely to need removal to repair some other portion of the printed circuit board 16, and it would be too costly and inefficient if all of the heat-sunk components (assuming that there are more heat-sunk components than only 18a) were to be damaged by the removal of the heat transfer plate for repair. Consequently, device 318 cannot be bonded to both the upper surface of component 18a and to the lower surface 316l of heat transfer plate 316. At least one of the thermal/mechanical connections must be readily breakable; this may be either the connection between device 318 and the lower surface 316l of top plate 316, or between the lower surface of device 318 and the upper surface of component 18a. In order to identify the breakable joint or connection, a heavy dash line 340 is illustrated at the upper surface of component 18a. Devices such as 318 tend to be flexible pads of various sorts, which must be somewhat compressed to provide adequate heat transfer, but which cannot be excessively compressed. These pads, in general, do not have thermal properties approximating those of a fusion bond or a metallic connection. In general, the tolerances will not allow the thinnest devices 318 to be used, which are simple interface layers of thermally conductive grease. Such greases, even though they may not have thermal characteristics matching those of metals, are so thin that their heat transfer performance is good.
Improved heat transfer arrangements are desired.